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"Scientists monitoring global atmospheric carbon dioxide concentrations report that, for the first time in human history, CO2 levels could soon rise above 400 parts per million for a sustained period of time in much of the Northern Hemisphere."

"Hourly readings have surpassed 400 ppm in the past week, but daily averages remain just below 400, reported The Guardian. Daily readings are expected to surpass 400 ppm in early May. They will reach their annual peak by mid-month.

The measurements come from the NOAA-operated Mauna Loa Observatory in Hawaii, which has maintained a continuous record of atmospheric CO2 concentrations since 1958. Records of earlier levels come from air bubbles inside Antarctic ice core samples."